Method for transmitting data packets on carrier frequency with linear variation and transmitter implementing this method

ABSTRACT

A method for transmitting data packets having a heading followed by a data field. The data of the field is transmitted by groups of symbols. According to the OFDM technique, subcarriers modulated by the groups are generated. A frequency modulation of a carrier signal delivered by a linear ramp generator is produced with a set of modulated subcarriers. When the signal is received, mixing with a linear ramp results in subcarriers which are separated and then demodulated for supplying data. This is useful for transmitting data packets particularly in wide band networks.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the transmission, in particular inbroadband networks, of data in packets, these packets comprising aheader at a first carrier frequency, followed by an information field ata second carrier frequency, the start of transmission of the field beingtied to the start of transmission of the header of the correspondingpacket.

2. Discussion of the Background

It is known practice to transmit data in packets each comprising aheader followed by an information field, the information afforded by theheader making it possible, among other things, for the packet to berecognized and taken into account by the parties for whom it isintended.

It is known practice, utilizing a given frequency band for transmittingpackets in a network, to define various transmission channels within theband and to allocate them to the transmitters of the network, either ina predetermined manner or on the basis of the transmission requirements;it turns out that such a method does not allow optimal use of thefrequency band and is scarcely practical especially when a receiver isliable to receive packets on any of the channels and must thereforemonitor the transmission of packets on all the channels.

It is also known practice to assign, within the band of transmissionfrequencies of a network, a channel to the headers; the receivers neednow monitor only this channel and the information provided by theheaders makes it possible to receive the fields, the latter beingtransmitted in the remainder of the useful band on a carrier frequencywhich may be constant or vary in a predetermined manner, for example injumps, throughout the duration of the packet.

SUMMARY OF THE INVENTION

The invention lies within the realm of the transmission of the field ona carrier frequency which varies. The purpose of the invention is toimprove the conditions of transmission, in particular as regards thespreading of the spectrum, the bit rate and the ease of exploitation.

This document deals with symbols. It is recalled that this involvesgroupings of binary information referred to as bits; these groupings maybe expressed under various forms of modulation (amplitude, frequency,phase), each value of the grouping being represented by a state of theconstellation in the complex plane. By way of example a grouping of m=3bits may be expressed through a modulation with N=2^(m)=2³=8 phasestates, each phase state being situated on the unit circle at multiplesof π/4. Additionally, more particularly, in the case of two-statemodulation, the symbol corresponds to one bit.

By using, for the information field, a signal at a carrier frequencywhich varies according to a linear ramp, this purpose is achieved bymodulating the said signal via several subcarriers rather than via onesubcarrier, these subcarriers being, themselves, modulated by the datato be transmitted according to a technique of orthogonal frequencydivision multiplexing, generally referred to as the OFDM technique afterits initials. It should be noted that, in what follows, the OFDMtechnique covers both straightforward OFDM and coded OFDM also referredto as the COFDM technique after its initials; it is recalled in thisregard that, considering a binary train, in the OFDM technique thesymbols are transmitted in groups of N symbols with N an integer greaterthan one, respectively on N subcarriers and during a time equal to thetime to receive the N symbols. It is recalled that the COFDM techniqueis merely a variant of the OFDM technique in the sense that, in theCOFDM technique, there is moreover associated a coding function whichmakes it possible to obtain, starting from the N input symbols, Noutputs each composed of a weighting of the N input symbols.

According to the invention there is proposed a process for transmittingdata in packets, these packets comprising a header at a first carrierfrequency, followed by an information field at a second carrierfrequency, the start of transmission of the field being tied to thestart of transmission of the header of the corresponding packet,characterized in that it consists, for the transmission of the field, inusing N, with N an integer greater than 1, distinct and simultaneoussubcarriers, in splitting the data to be transmitted in the field intosuccessive groups of N symbols, in assigning the N symbols respectivelyto the N subcarriers by OFDM multiplexing and in modulating these Nsubcarriers respectively by these N symbols so as to obtain a modulatingsignal made of the N subcarriers thus modulated, in generating a signalat the second carrier frequency varying over time according to a linearramp, in modulating the signal at the second carrier frequency by themodulating signal, and, on reception, in mixing the signal correspondingto the information field with a ramp-like signal similar to the signalat the second carrier frequency so as to obtain a signal correspondingto the modulating signal and to extract therefrom the data of theinformation field.

According to the invention there is proposed a transmitter forimplementing the process, characterized in that it comprises means forcomputing groups of symbols representative of the data to be transmittedin a field, first means of modulation by the OFDM technique forcomputing N, with N an integer greater than 1, subcarriers modulated bythe groups so as to generate a modulating signal, a ramp generator forgenerating a carrier signal whose frequency varies over time accordingto a linear ramp and second means of modulation for receiving thesignals generated by the first means of modulation and the rampgenerator and for performing a modulation.

According to the invention there is also proposed a receiver forimplementing the process, characterized in that it comprises a mixerwith a first input for receiving a signal transmitted according to theprocess, a second input and an output, a ramp generator for delivering,on the second input of the mixer, a signal whose frequency varieslinearly over time and, at the output of the mixer, a frequency/timeoperator followed by a subcarrier demodulation circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood and othercharacteristics will become apparent with the aid of the followingdescription and the corresponding figures which represent:

FIGS. 1 to 3, time charts relating to packets transmitted according tothe invention,

FIG. 4, the diagram of a transmitter according to the invention,

FIG. 5, the diagram of a receiver according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the diagrams, the devices for accurate synchronization, which comewithin commonplace technology, have not been represented so as to makethe drawings clearer and to simplify the account.

FIG. 1 is a diagrammatic chart showing how, within the realm of theinvention, two successive data packets are transmitted. Each packetcomprises a header E1, E2 followed by an information field C1, C2.

The header is transmitted on a fixed carrier frequency Fe and occupies atransmission channel whose limits are two frequencies F0, F1.

The information field is transmitted simultaneously in four subcarrierswhich modulate a carrier Fc and this carrier has a ramp-like variation,that is to say the value of its frequency is a linear function of timet; in FIG. 1 four ramps Fc+fo, Fc+2fo, Fc+3fo, Fc+4fo constitute asymbolic representation of each of the information fields C1 and C2.

The information field thus occupies a frequency channel of constantwidth, which moves continuously over time between the value F1 and avalue which, for the longest packets, is at most equal to F2, the bandF0-F2 being the frequency band allocated to the network in which therelevant packets are exchanged.

It should be noted that in the various charts of this document theproportions between the diverse time intervals and between the diversefrequency intervals are given merely by way of examples since theydepend on the applications; moreover, the proportions are not compliedwith between the various charts.

The transmission, in the network, of the data of the information fieldis carried out in groups; the modulation used on each subcarrier can beof various types; two-state phase modulations, referred to as DPSK afterthe initials standing for Differential Phase-Shift Keying, are thesimplest and most economical for transmitting packets of this type;modulations with more than two phase states, in particular with 8 phasestates, referred to as D8PSK, make it possible to attain higher bitrates but at the cost of inferior immunity to noise. In the case, forexample, of four subcarriers for the field, the groups are made up offour symbols which reduce to four bits in the case of a modulation withtwo phase states. These groups are transmitted by the straightforwardOFDM technique or the coded OFDM, that is to say the COFDM, technique,depending on the way in which it is planned to operate the network.

Thus, the overall bit rate is shared between the subcarriers and mayeasily be increased or decreased by increasing or decreasing the numberof subcarriers.

By way of example use has been made of a set of 16 subcarriers over atotal instantaneous frequency band of 8 MHz, each subcarrier making itpossible to transmit around 250 kbaud/s, i.e. in the case of QPSKmodulation, a bit rate of 500 kb/s per subcarrier and an overall bitrate of 8 Mb/s. Each symbol time therefore lasts 2 μs. With a ramp whichlasts around 500 μs for a total band of around 250 MHz the variation infrequency between the start and the end of a symbol time is then 1 MHz.

FIG. 2 is intended to show how, on account of its makeup, theinformation field is scarcely sensitive to fading. This is because, if afrequency band Ff1-Ff2 is blocked by selective fading, a symbol ofduration ts2-ts1, transmitted by modulation of a frequency ramp, isaffected by this fading only when passing through the band Ff1-Ff2, thatis to say only during the time tp2-tp1 and this time is inverselyproportional to the slope of the ramp used. It is thus apparent that theproblem of selective fading amounts to a problem of temporal fading, theeffects of which are then reduced by the modulation by the OFDMtechnique which makes it possible to lengthen the duration of the symboltime, for equivalent bit rate, with respect to conventional transmissionon a single subcarrier.

It should also be noted that, because the transmission energy isdistributed not over a single subcarrier but over several and becausethe carrier is modulated by a linear ramp, the transmission spectrum ofthe information fields is spread and undergoes a translationcorresponding to the modulation of the frequency ramp. This isillustrated by FIG. 3 which represents, at a given instant, theamplitude |A| of the main spectral lines corresponding to thesubcarriers, as a function of transmission frequencies; as the carrierfrequency varies between its two extreme values, the spectrum, whoseenvelope is depicted by a broken curved line, undergoes a shift depictedby a horizontal arrow in FIG. 3. The case represented in this figure isthat of eight subcarriers.

FIG. 4 is the diagram of a transmitter for implementing the process fortransmitting packets which has just been described with the aid of FIGS.1 to 3. The example described relates to transmission on eightsubcarriers by the COFDM or straightforward OFDM technique; in thediagram, a circuit 50, drawn dashed, represents the coding functionwhich makes it possible to pass from the OFDM technique to the COFDMtechnique.

The information to be transmitted is applied to the input of a shiftregister 1 whose eight outputs are linked to the input of a bufferregister 2. This information consists of symbols delivered at the rateof 8fo which are grouped together into successive groups of 8 symbolseach, by virtue of the shift register/buffer register assembly; theduration of a group is therefore 8T=8/8fo=1/fo.

The contents of the register 2 are applied to the first terminals of anelectronic switch 3 which, if it could be embodied in a mechanicalversion, would be an eight-wafer two-position switch.

In what follows it will firstly be considered that the switch 3 isconnected directly to the eight inputs of a digital generator, 5, ofeight modulated subcarriers, of respective frequencies fo, 2fo, 3fo, . .. , 8fo; these ratios between the frequency values are given by way ofnon-limiting example. In the case of the example described, where anoversampling by factor 8 of the fastest subcarrier is performed, thegenerator 5 comprises a trigonometric table sampled at the frequency64.fo in angular gaps of 2π/64 for the frequency fo, of 2π.2/64 for thefrequency 2fo, of 2π.3/64 for the frequency 3fo, . . . , of 2π.8/64 forthe frequency 8fo. In the example described each subcarrier is assigneda multiplicative coefficient +1 or −1, for two-state phase modulation,depending on whether the bit corresponding to the relevant subcarrierhas the value 1 or 0. The sampling is performed sine-wise andcosine-wise so as to generate two quadrature components for eachsubcarrier. The eight components I and Q are transmitted, respectivelyto two accumulators 6 a, 6 b in which the eight components of liketemporal index are summed to yield, as accumulator output, a binarynumber; in the example described this binary number is made up of eightbits. The accumulators 6 a, 6 b are linked respectively to twodigital/analog converters 7 a, 7 b working at the rate of 64.fo andfurnished with low-pass output filters.

An analog I and Q modulator 8 receives the signals from the converters 7a, 7 b on two first inputs and the signals I and Q from a digitalsawtooth generator with analog outputs on two second inputs. Themodulator 8 thus receives two modulation signals of the form sin a, cosb on its first inputs and two carrier signals of the form sin a, cos aon its second inputs; it carries out the operation cos a.cos b−sin a.sinb and therefore outputs a signal of the form cos(a+b), that is to say acarrier modulated by eight subcarriers. The output signal from themodulator 8 is next amplified, in a linear amplifying chain (notrepresented), before being transmitted.

The switch 3, according to FIG. 4, makes it possible to insert groups oftest data between the groups of information data originating from theregister 2. These groups of test data have a configuration which isknown to the receivers for which the data packets are intended; theymake it possible, in a conventional manner, to adjust the receivers soas to take account, in particular, of the pulse response engendered bythe diverse multipaths of the signal between a receiver and atransmitter.

In FIG. 4 a rectangle has been drawn dashed; this rectangle represents atransformation matrix 50 which is inserted between the switch 3 and thesubcarrier generator 5 when it is desired that the transmission beperformed not by the straightforward OFDM technique as was consideredwith the transmitter as described hitherto, but by the COFDM technique;the matrix 50 transforms the binary data of the input data group into aset of output signals with interdependent polynomials, so as to reduce,in a conventional manner, the transmission errors.

In the foregoing it was considered that the symbols of the groupsrepresented 0 or 1 bits but, of course, they may also represent valuesof modulation constellations such as modulations of phase, of frequencyor of phase, with N states; and the symbols then represent complexamplitude and phase values, it being possible for each symbol to bewritten in the form A_(k)e^(jφk) where A_(k) represents an amplitude, eis Euler's number, j is the imaginary unit, φk is an angle and k denotesthe symbol with 0≦k≦N−1, if N denotes the number of modulation states.

In the transmitter according to FIG. 4 each subcarrier is constructedtemporally, sample after sample, by taking account of the binaryinformation to be transmitted; this involves direct generation of thesubcarriers. A conventional alternative consists in producing thetransmitter in such a way as to compute the theoretical spectrum withthe modulated subcarriers and to perform an inverse discrete Fouriertransform, such as an Inverse Fast Fourier Transform or FFT⁻¹, so as togenerate temporal- signals, each of which is valid over the duration ofa group of N symbols.

FIG. 5 is the diagram of a receiver for receiving packets transmitted bythe transmitter according to FIG. 4.

In what follows, with regard to everything which is conventional in thetransmission of packets consisting of a header and an information field,in the technique of echo discrimination and in that of transmissionerror correction, the explanations will be given without being burdenedby the details of embodiment which are within the scope of the personskilled in the art so as to highlight better that which, in thereceiver, is specific to the invention.

The signal received by the receiver according to FIG. 5 is analysedcontinuously by a synchronization circuit 10. The circuit 10 comprises aclock which is synchronized with the headers of the packets; to do this,when a first correlation peak exceeds the detection threshold, theanalysis of the subsequent peaks, during a given time interval,preferably a symbol time, makes it possible to sharpen thesynchronization of the clock and thus to give an accurate time referencefor receiving the information field. When this synchronization isachieved, the signal received is mixed, in a mixer 11, with the outputsignal from a ramp generator 12 which supplies, on a single output, ananalog signal with the same slope and the same frequencies as the outputsignals from the slope generator 9 of the transmitter according to FIG.4; the ramp generator 12 is triggered coherently with thesynchronization pip obtained in the synchronization circuit 10. Themixer 11 comprises an output filter for eliminating the “sum frequency”terms.

A time-frequency operator 13 receives the signals from the mixer 11;this involves, in the example described, an operator of the Fouriertransform type, such as the Fast Fourier Transform or FFT. The operator13, also coherently with the synchronization pips computed by thesynchronization circuit 10, supplies, in the example described, a set of64 analysis signals making it possible to extract the informationcarried by the eight subcarriers of the information field signal. These64 analysis signals are supplied either to a circuit 15 for calculatingand controlling pulse response correction and frequency gaps when thedata transmitted are test data, or to a circuit 14 for demodulating thesubcarriers when the data transmitted are information data. The circuit15 supplies pulse response correction signals to the circuit 14 andfrequency offset correction signals to the ramp generator 12.

The next three paragraphs relate to remarks regarding the faults whichmay affect the transmission of data in a network implementing thetransmission process just described with, by way of example, atransmitter according to FIG. 4 and a receiver according to FIG. 5.

The multipaths between a transmitter and a receiver, in so far as thedifferences between their respective durations are short with respect tothe duration of a symbol, are not troublesome precisely because the OFDMtechnique is employed. It is however possible to estimate these multiplepaths so as either to choose, between the signals corresponding to thevarious paths, the one possessing the most energy, or to recombine allor some of the signals relating to the multiple paths so as to increasethe energy of the demodulated signal and thus improve the robustness ofthe process. Here this involves estimating the pulse response of thechannel, the components being used according to their amplitude andtheir phase to retrieve the original signal; strictly speaking, thisestimate must be remade along the whole ramp since the value of thecarrier frequency changes continually but in practice it is sufficientto make this estimate over a few regularly spaced portions of thefrequency ramp. To make this estimate it is possible, for example, touse test groups made up of 1 out of all the subcarriers and todetermine, through a frequency analysis of the output signal from themixer 11, the phase and the amplitude of each of the components of thespectrum, for each subcarrier.

It is also possible to estimate and to correct the frequency gap betweenthe transmitter and the receiver; this frequency gap is due to thedrifting which exists between the local oscillators of the transmitterand of the receiver, to the differences imparted by the Doppler effect,to the residual frequencies generated by imperfect synchronization withthe header; each of the subcarriers is affected by the same frequencydrift. To determine this drift so that it can be taken into account, itis possible, for example, to transmit at the start of the ramp orregularly within the ramp, a group of known symbols such as a series ofis so as to obtain a set of pure subcarriers which aids the detection ofthe frequency drift with the help of a spectrum analyser.

The invention is not limited to the description just provided andextends more generally to all variants within the scope of the personskilled in the art in particular as regards the circuits forimplementing the process, the frequencies used, the number ofsubcarriers of the information fields; it is obvious, moreover, that theheader may itself also be transmitted with the help not of a single butof several subcarriers and that the values of the frequencies of thesesubcarriers need not be regularly distributed.

What is claimed is:
 1. A process for transmitting data in packets, thepackets comprising a header at a first carrier frequency, followed by aninformation field at a second carrier frequency, a start of transmissionof the information field being tied to a start of transmission of theheader of the corresponding packet, comprising: using N distinct andsimultaneous subcarriers for transmission in the information field,where N is an integer greater than 1; splitting the data to betransmitted in the information field into successive groups of Nsymbols; assigning the N symbols respectively to the N subcarriers byOFDM multiplexing; modulating the N subcarriers respectively by the Nsymbols thereby obtaining a modulating signal made of the N subcarriers;generating a signal at the second carrier frequency varying over timeaccording to a linear ramp for optimizing bandwidth use; modulating thesignal at the second carrier frequency by the modulating signal; andmixing the signal, upon reception, corresponding to the informationfield with a ramp signal corresponding to the signal at the secondcarrier frequency to obtain a signal corresponding to the modulatingsignal and to extract from the signal data of the information field. 2.The process according to claim 1, wherein the modulating the Nsubcarriers comprises: constructing from a table of samples havingvalues representative of the N subcarriers modulated by the data of eachof said successive groups, thereby directly generating the subcarriers;summing samples of like temporal index; and converting a result ofsummation of the samples from digital to analog, thereby obtaining amodulating signal.
 3. A transmitter for transmitting data in packets,the packets comprising a header at a first carrier frequency, followedby an information field at a second carrier frequency, a start oftransmission of the information field being tied to a start oftransmission of the header of the corresponding packet, wherein thetransmitting comprises using N distinct and simultaneous subcarriers fortransmission in the information field, where N is an integer greaterthan 1, splitting the data to be transmitted in the information fieldinto successive groups of N symbols, assigning the N symbolsrespectively to the N subcarriers by OFDM multiplexing, modulating the Nsubcarriers respectively by the N symbols thereby obtaining a modulatingsignal made of the N subcarriers, generating a signal at the secondcarrier frequency varying over time according to a linear ramp,modulating the signal at the second carrier frequency by the modulatingsignal, and mixing the signal, upon reception, corresponding to theinformation field with a ramp signal corresponding to the signal at thesecond carrier frequency to obtain a signal corresponding to themodulating signal and to extract from the signal data of the informationfield, the transmitter comprising: a computing device, configured tocompute groups of symbols representative of data to be transmitted in afield; a first modulator, configured to generate a modulating signal byOFDM for the N subcarriers; a ramp generator configured to generate acarrier signal, wherein the carrier signal has a frequency variant overtime according to a linear ramp for optimizing bandwidth use; and secondmodulator, configured to receive signals generated by the firstmodulator and the ramp generator.
 4. The transmitter according to claim3, wherein the first modulator comprises: a digital generator ofmodulated subcarriers; a summation device; and a digital to analogconverter, wherein the digital generator, the summation device, and thedigital to analog converter are arranged in series.
 5. A receiver fortransmitting data in packets, the packets comprising a header at a firstcarrier frequency, followed by an information field at a second carrierfrequency, a start of transmission of the information field being tiedto a start of transmission of the header of the corresponding packet,wherein the transmitting comprises using N distinct and simultaneoussubcarriers for transmission in the information field, where N is aninteger greater than 1, splitting the data to be transmitted in theinformation field into successive groups of N symbols, assigning the Nsymbols respectively to the N subcarriers by OFDM multiplexing,modulating the N subcarriers respectively by the N symbols therebyobtaining a modulating signal made of the N subcarriers, generating asignal at the second carrier frequency varying over time according to alinear ramp, modulating the signal at the second carrier frequency bythe modulating signal, and mixing the signal, upon reception,corresponding to the information field with a ramp signal correspondingto the signal at the second carrier frequency to obtain a signalcorresponding to the modulating signal and to extract from the signaldata of the information field, the receiver comprising: a mixer,comprising: a first input for receiving a signal; a second input; and anoutput; a ramp generator, wherein the ramp generator is configured todeliver a signal having a frequency variant over time according to alinear ramp, for optimizing bandwidth use, to the second output of themixer; a frequency time/operator; and a subcarrier demodulation circuit.